Here is an overview of the Frsky R9M-Lite module, and we will test the latency and see how it compares to the TBS Crossfire. Both are great 900MHz RC systems for long range and many are hesitating which one to get. With latency being one of the main considerations we hope you find the test useful.
We previously reviewed the R9M module, which is for the radios with JR module bay such as the Taranis X9D-Plus, Q X7 and Horus X10S. The R9M-Lite module is the miniature version which is designed specifically for the Taranis X-Lite radio.
Where To Buy?
- Amazon – https://amzn.to/2VnB3Ay
- Banggood – http://bit.ly/2LLMLAr
- GetFPV: http://bit.ly/2RwrqQM
What is the R9M-Lite Module for?
The R9M-Lite is an affordable long range option compared to the Crossfire. It’s easy to install on the X-lite without doing any mods, basically just plug and play! The X-Lite actually support Crossfire too but it requires special mounts.
900MHz radio system provides a much more solid and reliable link compared to the built-in 2.4GHz RF module in the X-Lite. You can fly around big fields and not have to worry about losing signal. You might even be able to go behind obstacles you could never dream of with 2.4GHz.
Radio receiver options are also pretty good, the R9-mini and R9MM receivers are even smaller than the 2.4GHz R-XSR, even better at half the price! But just like any 900MHz systems, antenna mounting is different than 2.4GHz. You need to mount them in a specific way to have the best possible signal.
Installation and Setup
The way we hold the transmitter will have the antenna pointing directly at the aircraft which gives a weak signal. You might want to get a 45 degree or even a right-angle RP-SMA adapter.
- 45-degree – http://bit.ly/2uBdLf2
- 90-degree – https://amzn.to/2Rv67iA
Flash the correct firmware on your radio and RF module (and receiver), and select the correct mode for your region. Using the correct frequency for your region is critical to signal reliability.
Here is a screenshot of the Model Setup in OpenTX.
Latency in our Radio Control System
The latency test was done by Andrey Mironov, with his permission to share the result here. You can learn more about the TBS Crossfire in this article. Check out our review of the Crossfire vs. R9M.
The latency in our RC link can be divided into 4 parts:
Gimbals => TX Interface (OpenTX) => TX module => RX => FC
The latency is measured between the points before OpenTX and after RX (bold text).
Measuring Latency in R9M-Lite and Crossfire
The latency are measured with the TX modules installed in the Frsky X-Lite. Other gear used:
- R9M-Lite TX module with R9 Mini receiver
- Crossfire Micro TX module with Crossfire Micro V2 receiver
Further Reading: Does the Frsky X-Lite support Crossfire?
Note that there is a stick filtering (6-sample averaging) that exists in OpenTX, which introduces a varying delay. To properly measure latency a custom version of OpenTX was built to get rid of this filtering. This was explained in more detail in our previous latency test of the X-Lite.
First test consisted of tapping into the gimbal output line and generating a 260Hz PWM signal with a STM Discovery board, altering between 10% and 90% duty cycle every 128 milliseconds, while observing the resulting receiver output signal frame over the course of 60 seconds.
Latency is measured between the falling edge of gimbal PWM signal and the start of corresponding RX signal frame.
Note that this doesn’t take RX signal frame length into account. The frame length for SBUS is slightly less than 3 ms, that of CRSF is 0.7ms, which is in turn dictated by 100k vs. 420k baudrate, 2.97ms × 100/420 = 0.7ms.
The R9M-Lite is flash with FCC firmware.
Results
Here is the average latency:
- R9M-Lite => SBUS – 14.07ms
- Crossfire Micro TX => CRSF – 13.9ms
- Crossfire Micro TX => SBUS – 20.23ms
The R9M-Lite actually performed surprisingly well, very close to the Crossfire in terms of latency. But note that SBUS protocol has a longer frame length than CRSF protocol – over 2ms.
Another surprising observation is that the Crossfire doesn’t seem to work as well with SBUS, there is an 8ms extra delay compared to when using CRSF protocol on the receiver. Our guess is that the Crossfire goes into 50Hz mode when SBUS is selected as output (instead of 150Hz). However R9M has 150Hz for SBUS.
Here is the full result:
| Latency (ms) | Mean | Stddev | Min | Max | 25% | 50% | 75% | 95% | Refresh rate (Hz) | Notes |
| R9M Lite – SBUS (R9 Mini) | 14.066 | 2.501 | 7.515 | 20.205 | 12.140 | 14.524 | 16.016 | 17.843 | 150 | add 2.97 ms for SBUS |
| XFIRE Micro – CRSF (Micro V2) | 13.900 | 2.448 | 8.432 | 19.635 | 12.075 | 13.879 | 15.752 | 17.842 | 150 | add 0.7 ms for CRSF |
| XFIRE Micro – SBUS (Micro V2) | 20.231 | 6.233 | 7.680 | 32.831 | 14.828 | 20.606 | 25.346 | 29.756 | 50 | add 2.97 ms for SBUS |
| OpenTX latency (switch) | 5.626 | 1.478 | 2.046 | 8.937 | 4.628 | 5.630 | 6.574 | 7.558 |
All captures for Saleae as well as Python script used to produce the results will be posted on Andrey’s GitHub.
Edit History
- Jul 2018 – Article published
- Jan 2019 – Added overview of R9M-Lite and how to setup
26 comments
I wonder how the overall latency is. rx + camera + goggles + …
Cheers
as for the d16 on r9m if I change from 8 channels to 16 does latency increase?
it’s SBUS, so yes
TBS announces a new CRSF shot protocol. They say it’s up to 25% faster than the previous CRSF.
(team-blacksheep.freshdesk.com/support/solutions/articles/4000153333-crsf-shot-open-tx)
And DJI says that fast band SBUS (HDL) is 7ms.
That’s about 23% faster given the previous SBUS was 9ms at 8ch. I think both protocols are faster in a similar way.
Anyway … I’m curious about the latency of CRSF shots and fast band SBUS. I hope you test this.
I used Google Translator. Thank you.
Honestly… who’s going to notice a few ms of latency difference… :D
it would be nice to compare the latency of the r9m lite with accst firmware and with access to understand if there is actually less latency ….
Any chance you can do an update with ACCESS 2.4 and 900mhz vs crossfire?
Hi,
Good work, but for close source systems we never know what tricks are done do keep time on air lower.
First of all, when both tx and rx are close each other, different bandwidth (limited rx sensitivity) may be used to speed up transmission. Another problem is that you are changing only single channel value… there could be logic, that transmit first changed channels values, to limit air frame size…. when you will try change all 16 channels, the lag may be bigger cause it must transmit more data… next thing, some kind of “compression” may be applied, like delta transfering, etc… so for different changes, different amount of data may be transfered…
So in real noisy RF environment, it may look different :)
Regards
Kris
X-Lite Pro is now sold, with improved latency they say as well as r9m lite Pro with 1w power! :D Any plans too test it? mines are on the way already hehe
Nice comparison. Full size r9m has more latency? For not so long range but freestyle flying and racing the r9m lite is better than the full size one?
ok, R9mLite is good if we speak about latency but r9m lite has less output power than it told in specs.
one of the tests on youtube got only 60 m-watts instead of 100.
another one disappointing thing is that r9m lite could not be moded to get more than 100 m-watts like crossfire up to 250.
but I dont think r9m-lite is bad transciver. it gives you possibility to fly over the obstacles with a little more range as 2.4ghz but without obstacles.
It would be great to know the latency of the r9m system too.
I agree. Did you find it anywhere?
Hi,
are there two different hardware versions of the R9M lite module (LBT / FCC)? Or is the FCC module only differentiated from the LBT module by the software?
only different in software.
Found the answer: Testing Methods and Full Result
For the sake of testing, Andrey built a custom version of OpenTX which doesn’t do this 6-sample filtering. He was testing with the built-in iXJT module in the X-Lite, and an XSR receiver with the latest EU LBT firmware, in D16 8ch mode with telemetry on.
Thanks for checking Greg!
But I’m still wonder can I use it more than 8 channels without (a bit) latency penalty :)
Is R9M still limited to 8CH for lower latency like XSR or XM?
good question, were these test conducted with 16 channels and telemetry?
Would like to know if the R9M module has the same latency?
Great comparison!! :)
Can you please check the R9M lite latency VS the Crossfire
But with Fport on the R9M lite?
It is said that Fport is 3-4ms less latency on X4R here:
youtube.com/watch?v=hCX2Qi51ZtU
The only reason that is this way is because FPORT firmware is much newer than ordinary SBUS firmware for X4R.
There won’t be any difference for R9.
Similar question would this suggest the R9M would perform as well as the R9M-Lite. I’m interested because if so I believe the trainer port on my Spektrum DX6 is SBUS and this may be the solution I need. Instead of either buying a new DX9 $450 & CRSF $225 in order to get 900Mhz but keep the high 150hz refresh for the lower latency.
No, this test doesn’t conclude anything about R9M.
No that’s not the ticket… DX9 can be moded very easy to use CRSF but without telemetry function, and Spektrum trainer port is PPM so added latency is in effect. I’m waiting on the i12 to mature for XFire.
Any comparison or test of the normal R9M module?